Electron beam tube focusing device



July 4, 1961 sUsuMu YAsuDA 2,991,382

ELECTRON BEAM TUBE FOCUSING DEVICE Filed March l1, 1959 S. Y SUDA \3 By W w O O O A ltorn ey nited States Patent O ELECTRON BEAM TUBE FOCUSING DEVICE Susumu Yasuda, Tokyo, Japan, assignor to Nippon Electric Company Limited, Tokyo, Japan, a corporation of Japan Filed Mar. 11, 1959, Ser. No. 798,767 xClaims priority, application Japan Mar. 20, 1958 3 Claims. (Cl. 313-84) It has been publicly known that use of a periodic magnetic field for focusing an electron beam in an electron tube is practical and has the further advantage of reducing the size and weight of the apparatus as compared to cases in which a solenoid coil or a permanent magnet for producing a uniform magnetic field is used.

Many treatises covering the theoretical as well as the practical aspects of this problem have recently been published.

However, a simple, and yet substantial method of compensating for discontinuities in cases where the input or output of the electron tube is to be derived from a circuit having dimensions of which widths cannot be ignored as compared to a half period of the periodic magnetic focusing field like a waveguide, etc., there seems to have been no appropriate solution put forth.

This invention relates to means for producing continuous periodic magnetic fields even if the magnetic focusing device in an electron tube is designed to be discontinuous at the input or output R.F. circuit.

According to this invention, discontinuities due to the presence of an R.F. circuit is compensated by the cornmon magnetic field produced by two cylindrical magnets with an R.F. circuit placed between them.

These magnets are positioned so that opposite poles may face each other. The means utilized may have a disadvantage when applied to the present system in that the intensity of the available magnetic field is relatively weak, on the other hand the simplicity of the magnetic focusing system and the further advantage of the structure being compact and light in weight, greatly outweigh this disadvantage. This system as compared to the conventional systems in which a uniform magnetic field at the electron gun or collector portion ensures against discontinuities due to the existence of the R.-F. circuit is extremely adaptable, for example, to small or medium power output traveling-wave amplifiers of so-called package-type in which a focusing system, tube envelope, and input and output waveguides are assembled integrally.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the `following description of an embodiment of the invention taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a portion of a magnetic focusing device incorporating the features of this invention;

FIG. 2 is a similar sectional view of a further modified form of this invention; and

FIG. 3 is a schematic diagram of magnetic shunt which controls the amount of magnetic ux passing through the above-mentioned two magnets.

Referring now to FIG. l, part A of the figure shows a magnetic structure to compensate for the discontinuities due to the presence of the R.F. circuit, consisting of two cylindrical magnets 1 and 2 with an R.F. circuit 3 placed between them.

These two magnets 1, 2 are positioned so that opposite poles face each other. These magnets being further combined with pole pieces 4 and S with the external projections for controlling the amount of magnetic flux passing through the two magnets in common, and pole pieces 6 and 7 controlling the distribution of magnetic flux density on the axis to a desired shape. The spacing d1 between both pole pieces 6 and 7 is equal to the outer or inner width of the R.F. circuit which is engulfed in a magnetic field. It will be seen from this structure that an approximate continuous periodic magnetic field may be produced on the axis of the apparatus, provided that the spacing 1 between pole pieces 4 and 5 is so adjusted as to make the maximum value of magnetic field intensity on the axis of the R.F. circuit conform to those on the axis of local magnetic field of each of the two magnets and on the axis of the conventional periodic magnetic structure shown at part B of FIG. l.

Details of the distribution of the above-mentioned magnetic flux may be understood more clearly by the following explanation.

Suppose that a continuous periodic magnetic field has been obtained by the method as shown in FIG. l.

A similar treatment as done by Dr. Chang (Optimum Design of Periodic Magnetic Structure for Electron Beam, RCA Review, p. 65, March 1955) will pe possible about the magnetic flux density (Bzo )on the axis, the total amount of magnetic flux 301 passing through the mid-plane of each paired pole pieces (z is taken to be zero at the plane) within the range R R1 (R1 is inner radius of the pole pieces), and the total amount of leakage magnetic flux p2 passing through the plane zl=0 within the range R R2 (R2 is outer radius of the pole pieces and magnets), provided that the increase of stray magnetic flux outside of the magnet due to the external projections of pole pieces 4 and 5 in FIG. l is neglected.

With part A in FIG. l, due consideration must be given not only to the aforementioned 1,01 and 1,02 but also to the stray flux N,b3 existing between a pair of pole pieces of each of the two magnets (within a range R1 R R1, where R1 is inner radius of the ring magnets) and the common magnetic flux 300 between the confronting surfaces of the magnetic pole pieces 6 and 7 (within a range R1 R R2l I-f it is assumed that the magnetic flux density Bz is distributed uniformly at z=0, an approximate relation L/2Bz=,u0. Zbo is established between said Bz and the magnetic potential i950 on the magnetic pole pieces, where no is permeability of vacuum and 2 0 is magnetic potential difference of the paired pole pieces.

As a result 1//0 will be expressed as follows:

R 4; loefRfazfRdRqo ff'taa- R12) Following a similar procedure as this for the local stray flux, we have Then the total amount of magnetic flux passing through where From these equations and doing suitable normalization similar to that performed in Dr. Changs article, we have suchequations as shown in Table 1.

andvBzo denotes the maximum value of the magnetic flux density of the periodic ield distributed on the axis.

These equations enableusrto perform the design of the magnetic circuit shown in part A of FIG. l. To make it clear, a numerical example will be presented hereunder. Let the value of electron beam current be 0.022A, beam potential V be .3000 V., and the radius of the beam 7 104 m.

Then We have Brillouin field Bb =238 gauss. If it is considered that the magnetic eld intensity is higher than the aforementioned theoretical Value by 50 percent from the practical point of view, we obtain an approximate value of 510 gauss for the peak ilux density ZO of the periodic magnetic focusing system. In this case the period of this system must be for a and P, respectively.4 If Vl which is limited by the 4 external diameter of the envelope of an electron tube is assumed to be 0.26 (13 mm. in diameter), we obtain from Eq. 1

BZ mil-5:32a or Hm=j640 oersteds If Bm of the magnet material used is 1490 gauss corresponding to the above-mentioned Hm, -m can be calculated to be 2.3.

From Eq. 2 we obtain i@1:016 for 1=0.26. Therefore, from Eq. 7, ,72:126 iig-0.24 (Eq, s).

From Eqs. 3 and 8 We obtain 1 i`2i.-O.78,Ythat is Rzl2l mm., and from P=0.4 the length of the magnet becomes 10 mm., while from q=0.96 the internal diameter is available as 15 mm.

On the basis of the aforementioned design procedure, it has been verified by experiment that the magnetic circuit structure combined with the waveguide having internal wall width in the axial direction of 6 mm. can afford the recurrent peak magnetic flux density of 500 gauss when the conliguration of pole pieces 4 and 5 is suitably designed.

As to the verification of the compactness and the light- Weight of the electron tube device to which this invention is applied, it will be enough to say that the total Weight of a recently developed 10 watts 6 kmc. package type travelling-wave amplifier applying the principles kof this invention was only 25kg. which included the focusing system, input and output-waveguides, the tube, radiator, and the cases to pack the above-mentioned elements.

The focusing system of the said amplifier has produced the recurrent peak magnetic flux density of around 650 gauss and a current transmission of more than 98 percent for a beam current of 30 milli-amperes through a helix which has an inner diameter of 2.5 mm.

In FIG. 2 there is shown a magnetic assembly substantially similar to that shown in FIG. 1. However, in this construction supplemental magnets 8, 9 etc. are placed on the R.F. circuit 3 so that the same poles may face the aforementioned cylindrical magnets 1 and 2. In this case, the common compensatory magnetic flux for the R.F. circuit produced by magnets 1 and 2 can be added to the magnetic flux from said supplemental magnets, and even higher recurrent peak magnetic field may be obtained along the axis of the focusing structure as compared with that of the structure shown in FIG. 1.

Such a focusing device as shown in FIG. 2 is especially suitable to be used in relatively high frequency microwave electron device, for example, an X-band packagetype travelling-wave ampliiier, since dimensions of the R.F. circuit becomes suitably small to apply the principles of this invention.

In FIG. 3 there is shown the configuration of a magnet ic shunt which is suitable to be used as an outside extension part of the pole pieces 4 and 5 of FIG. l or FIG. 2.

Reference characters 1 and 2 denote two cylindrical magnets with R.F. circuit 3 (a waveguide in this figure) placed between them. 10 and 11 of this figure are ironmade wings fixed to magnetic or non-magnetic support rings 12 and 13 at one end and magnetically connected with the magnets 1 and 2 at another end, respectively. These wings are bridged by iron bars 14 over the said magnets 1 and2 and R.F. circuit 3. The amount of the aforementioned common magnetic lux passing through magnets 1 and 2 is arbitrarily controlled by adjusting the number of said iron bars 14.

With this type of construction, the increase of stray magnetic ux outside of the magnet (il/2) due to external projections of pole pieces can be kept to a minimum, since the confronting surfacearea between opposite poles Aof each magnet may lbe minimized.

While Ihave described above the principles Vof myiu-Y vention in connection with specific embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. An electron beam focusing device comprising a pair of cylindrical magnets positioned with unlike poles opposed to one another, an R.F. circuit disposed between said pair of cylindrical magnets, irst and second annular pole pieces, said iirst annular pole pieces concent-ric with s-aid cylindrical magnets at the extreme ends of said pair of cylindrical magnets and extending inwardly of said cylindrical magnets so las to be magnetically combined with said second annular pole piece to produce magnetic iields of a half period of the periodic magnetic field on the axis of each portion of said cylindrical magnets, second annular pole pieces positioned at either side of said R.F. circuit and at the adjacent ends of said cylindrical magnets and concentric therewith, said first annular pole pieces extending outwardly of said cylindrical magnets and shunting the magnetic flux produced by said cylindrical magnets so as to produce a magnetic ield of a half period of the periodic magnetic field on the axis of said R.F. circuit, and said rst Iand second annular pole pieces providing approximately continuous periodic mag- 6 netic elds over one and a half periods together with said pair of cylindrical magnets.

2. An electron beam focusing device as in claim 1 further comprising a pair of permanent magnets placed on opposite sides of said R.F. circuit and positioned with their poles opposing the poles of said pair of cylindrical magnets.

3. An electron beam focusing device as in claim 1 further comprising a cylindrical frame of magnetic material having annular members positioned concentric with and spaced from said rst annular pole pieces, radial members connecting said annular members with said pole pieces and a plurality of bars of magnetic material positioned parallel to the axis of said magnet and connecting said annular members whereby the number of said bars will substantially reduce the magnetic ilux leakage outside of said cylindrical magnets.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,447 Hael et al. Dec. 17, 1940 2,305,884 Litton Dec. 22, 1942 2,867,745 Pierce Ian. 6, 1959 2,871,395 Cio Ian. 27, 1959 2,882,439 Nishio et a1. Apr. 14, 1959 

